The present invention relates to an epoxy resin composition useful in an electrical laminate. More particularly, the present invention relates to an epoxy resin composition for use in an electrical laminate, which exhibits low viscosity when dissolved in an organic solvent and good heat resistance when cured.
Epoxy resins have been used as materials for electrical/electronics equipment, such as materials for electrical laminates because of their superiority in heat resistance, chemical resistance, insulation property, dimensional stability, adhesiveness and the like. Most electrical laminates have been made from a varnish comprising a brominated epoxy resin, prepared from the reaction of a certain amount of diglycidyl ether of bisphenol A and a certain amount of tetrabromobisphenol A, dissolved in an organic solvent.
In general, to provide sufficient flame retardancy, the brominated epoxy resins used for preparation of laminates contain 20 to 23 weight percent of bromine. Thus, the conventional brominated epoxy resins usually have a tetrabromobisphenol A content of 34 to 39 weight percent. For this reason, it has been considered difficult to decrease the viscosity of a solution of such brominated epoxy resin by using a brominated epoxy resin having low molecular weight.
Further, the electrical laminates prepared from the brominated epoxy resins as described above have low glass transition temperature (Tg), e.g., 125.degree. C., and thus are not good for practical use. To improve heat resistance, approximately 15 parts by weight of cresol epoxy novolac resin is added to prepare an electrical laminate having a glass transition temperature of about 135.degree. C.
For ease of handling of an epoxy resin during preparation of prepreg, a solution containing the epoxy resin should preferably have lowest possible viscosity, but use the least amount of an organic solvent as possible. However, as mentioned above, it is difficult to sufficiently decrease the viscosity of the conventional epoxy resin solutions or difficult to decrease the amount of the organic solvent employed.
Furthermore, it is desirable to improve the heat resistance of the electrical laminates without using a relatively expensive resin, such as cresol epoxy novolac resin, if possible.
In view of the deficiencies of conventional epoxy resin compositions, it would be desirable to provide an epoxy resin composition useful in electrical laminates, which exhibits low viscosity when dissolved in an organic solvent and good heat resistance when cured.